The optically anisotropic layer is formed by aligning discotic liquid crystalline molecules (disc-like liquid crystalline molecules) and fixing the aligned state. The discotic liquid crystalline molecule generally has a large birefringence, and the discotic liquid crystalline molecules are aligned in various modes. When a discotic liquid crystalline molecule is used, an optical compensatory sheet having optical properties unobtainable by conventional stretched birefringent film can be produced. Molecular Crystals and Liquid Crystals, Vol. 84, page 193 (1982) discloses a triphenylene-based discotic liquid crystalline molecule having a negative birefringence. In order to use this liquid crystalline molecule for the optical compensatory sheet, the entire molecule constituting the optically anisotropic layer must be uniformly aligned, that is, the discotic liquid crystalline molecules are preferably oriented in a monodomain alignment. However, conventional discotic liquid crystalline molecules are oriented in a dual-domain alignment, and alignment defects are generated at the boundary of domain. Therefore, in many cases, conventional discotic liquid crystalline molecules cannot assure optical properties necessary for the application to an optical compensatory sheet. The optical properties are dependent on the chemical structure of the discotic liquid crystalline molecule. In this respect, many kinds of discotic liquid crystalline molecules have been studied and developed so as to obtain necessary optical properties. For example, JP-A-8-50206 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes to use an optical compensatory sheet having an optically anisotropic layer containing a discotic liquid crystalline molecule on a transparent support.
JP-A-7-306317 and JP-A-9-104866 disclose 2,3,6,7,10,11-hexa{4-(6-acryloyloxyhexyloxy)benzoyloxy}triphenylene as a discotic liquid crystalline molecule suitable for the formation of an optically anisotropic layer of an optical compensatory sheet. Incidentally, the retardation value (Δnd) of the optical compensatory sheet is determined according to the optical properties of a liquid crystal cell to be compensated. The retardation value (Δnd) is a product of the refractive index anisotropy (Δn) of the optically anisotropic layer and the thickness (d) of the optically anisotropic layer. When the refractive index anisotropy (Δn) of the optically anisotropic layer is large, the liquid crystal cell can be compensated even if the thickness (d) of the layer is small. However, it is very difficult for the discotic liquid crystalline compounds described in JP-A-7-306317 and JP-A-9-104866 to form an optically anisotropic layer having a sufficiently large refractive index anisotropy (Δn). JP-A-2001-166147 discloses a discotic liquid crystal having a large refractive index anisotropy, but the wavelength dispersion property is worsened, that is, the wavelength dispersibility is large, and the improvement of performance is not satisfied. In general, there is a trade-off relationship between wavelength dispersion property and refractive index anisotropy, and if the refractive index anisotropy is made large, the wavelength dispersion property deteriorates. This deterioration of wavelength dispersion property disadvantageously leads to the worsening of color tint change in color display, which is one of performances of the optical compensatory sheet. Therefore, a technique of overcoming the trade-off such that when the refractive index anisotropy is made large, the wavelength dispersion property deteriorates, has been demanded.
It is known that the discotic liquid crystal phase can be roughly classified into a columnar phase where center cores of no discotic molecules are stacked in a columnar state by the effect of intermolecular force, a discotic nematic phase (ND phase) where discotic molecules are disorderly aggregated, and a chiral discotic nematic phase. As described in W. H. de jeu, Physical properties of liquid crystalline materials, Gordon and Breach, Science Publishers (1980), the columnar nematic phase is often found, but the discotic nematic phase is scarcely found. Furthermore, with respect to the triphenylene compound, the discotic nematic phase is found only for compounds where the 2-, 3-, 6-, 7-, 10- and 11-positions are substituted, for example, by a substituted benzoyloxy group or a cinnamoyloxy group.